In general, a Doherty power amplifier has a structure of a carrier and a peaking amplifier connected in parallel by using a quarter-wave transformer (λ/4 line). Further, the Doherty amplifier is driven by a method in which the peaking amplifier controls a load impedance of the carrier amplifier by increasing the amount of current supplied to the load from the peaking amplifier as the power level is increased, thereby improving efficiency.
A microwave Doherty amplifier was first proposed by W. H. Doherty in 1936, and had initially been used in an amplitude modulation (AM) transmitter of a broadcasting apparatus using a high-power low-frequency (LF) vacuum tube or a medium frequency (MF) vacuum tube. Since then, there have been a variety of proposals for implementing the microwave Doherty amplifier with a solid-state device without using a vacuum tube, and numerous researches have been conducted to implement the proposals.
A Doherty amplifier employing an asymmetric power driving method has achieved high efficiency and linearity. Especially, the Doherty amplifier employed in base stations and handsets for mobile communications has been highly enhanced in performance. Such a Doherty amplifier for the high frequency range includes a power divider; a transmission line for synchronizing phases between a carrier amplifier and a peaking amplifier; the carrier amplifier and the peaking amplifier capable of outputting a maximum power by configuring the input matching circuits and the output matching circuits of the respective amplifiers such that the output level of the carrier amplifier is same as that of the peaking amplifier; an offset line for generating a proper load modulation by increasing an impedance output while the peaking amplifier is not operating; and quarter-wave transmission lines for performing the Doherty operation.
In this configuration, by sequentially providing the output matching circuits and, in turn, the offset line at output ends of transistors in the carrier amplifier and the peaking amplifier, an imaginary part as well as a real part can be matched, thereby enabling the Doherty operation while obtaining the maximum output power. (See Y. Yang et al, “Optimum Design for Linearity and Efficiency of Microwave Doherty Amplifier Using a New Load Matching Technique,” Microwave Journal, Vol 44, No. 12, pp. 20-36, December 2002.)
Further, an N-way Doherty amplifier having an optimum design for efficiency and linearity while improving a typical Doherty amplifier was also studied (See, Y. Yang et al, “A Fully Matched N-way Doherty Amplifier with Optimized Linearity,” IEEE Trans. Microwave Theory and Tech., Vol. 51, No. 3, pp. 986-993, March 2003.)
Moreover, there is presented an N-stage Doherty amplifier for gradually achieving high efficiency from a much lower power level in comparison with a conventional Doherty amplifier (See N. Srirattana et al, “Analysis and design of a high efficiency multistage Doherty amplifier for WCDMA,” EuMC Digest 2003, Vol. 3, pp. 1337-1340, October 2003.)
Further, in order to solve a problem in which the Doherty amplifier do not produce a maximum power output due to a low bias when the Doherty amplifier is implemented by using a solid-state device, there has been proposed a Doherty amplifier by using an envelope tracking device (See Y. Yang et al, “A Microwave Doherty Amplifier Employing Envelope Tracking Technique for High Efficiency and Linearity”, IEEE Microwave and Wireless Components Letters, Vol. 13, No. 9, September 2003., and J. Cha et al, “An Adaptive Bias Controlled Power Amplifier with a Load-Modulated Combining Scheme for High Efficiency and Linearity”, IEEE MTT-S Int. Microwave Sympo. Vol. 1, pp. 81-84, June 2003.)
However, even in the proposed Doherty amplifier, an additional device for controlling the power level of the peaking and the carrier power amplifiers is still required in order to achieve an improved linearity and the maximum output. To solve this problem, a research has been made about a Doherty power amplifier using an uneven power drive in which input powers are fed unevenly (J. Kim et al, “Optimum Operation of Asymmetrical-Cells-Based Linear Doherty Power Amplifiers-Uneven Power Drive and Power Matching,” IEEE Trans. Microwave Theory Tech., Vol. 53, No. 5, pp. 1802-1809, May, 2005; J. Kim et al, “Advanced Design Methods of Doherty Amplifier for Wide Bandwidth, High Efficiency Base Station Power Amplifiers,” 35th European Microwave Conference, Paris, France, pp. 963-966, October, 2005).
As discussed above, there have been developed various techniques for the Doherty power amplification. However, as base stations for mobile communications and terminal systems are being further reduced in size while there are increasing demands for saving the cost, a power amplification of a higher efficiency than the conventional Doherty power amplifiers is required.